1. Field of the Invention
The subject disclosure relates to a process for manufacturing membranes and membrane products, and more particularly, to a process for using polymer crosslinking to impart desirable characteristics to polymeric membranes and the membrane products therefrom.
2. Background of the Related Art
Microfiltration polymeric membranes are currently widely used in many industries for processes and applications such as filtrations, separation and concentration of solutions and suspensions. These membranes are fabricated by a phase inversion process.
By the term “phase inversion process,” we mean a process of exposing a polymer solution to a controlled environment to induce liquid-liquid demixing so as to form a pore structure. Phase inversion is a necessary step in the formation of microporous membrane. The process is induced by a number of mechanisms well known to those versed in the art. Examples of phase inversion include, but are not limited to: contacting the polymer solution coating to a solution of solvent and nonsolvent containing a higher percentage of nonsolvent than the polymer solution solution; thermally induced phase inversion; and exposing membrane to a vapor interface and evaporating the solvent from the polymer solution coating.
The effectiveness of these membranes is significantly limited by fouling at the membrane surface and pores, which, among other things, increases the pressure drop, decreases the permeate flux and changes solute selectivity over time. In certain applications, such as those which involve oil or protein containing solutions, membrane cleaning and replacement costs associated with fouling can increase the process operating costs to the point at which it becomes a significant economic burden to using membrane technology.
A major cause of fouling in these applications is due to the attraction of organics in the solution to the hydrophobic materials (i.e., “protein binding”) from which these membranes are typically fabricated. Protein binding often poses a more serious dilemma which causes even greater economic harm than the issues associated with membrane fouling due to loss of important proteins to the customer, especially when the binding occurs at lower concentrations.
The hydrophobic materials used in membrane manufacture typically possess low surface tension values and lack active groups in their surface chemistry for formation of “hydrogen-bonds” with water. Membranes fabricated from such materials have low wettability, that is, a high surface tension liquid, such as water, is not adsorbed into the membrane and instead tends to form discrete droplets on the membrane surface, without sufficient pressure in the system.
In contrast, membranes constructed of hydrophilic materials typically possess a high surface tension value and have the ability to form “hydrogen-bonds” with water, which results in the formation of a water film on the membrane surface. In addition, fouling of membranes prepared from hydrophilic materials is less severe and often reversible but these membranes have comparatively poor mechanical and thermal stability, and may be susceptible to chemical reactions with the process solution.
Membranes constructed of nylon are an exception to the above, in that nylon membranes are hydrophilic but they have a high tendency to bind proteins, which is probably due to the presence of amide and amine groups in the nylon chemical structure.
Hydrophobic membranes are typically used as sterile air filters where hydrophilic membranes could not function, whereas hydrophilic membranes are overwhelmingly preferred for aqueous applications. There is still a desire to impart a hydrophilic surface to naturally hydrophobic membranes to exploit other desirable properties such as excellent chemical resistance, along with desirable thermal and mechanical properties, which still makes using them more cost-effective than membranes constructed of hydrophilic materials regardless of the aforementioned benefits.
Since the properties of hydrophilic materials are more desirable than the properties of hydrophobic materials (at least for use in aqueous applications), it is the current practice in the art to modify polymeric membranes constructed of materials which are normally hydrophobic, such as polyvinylidene fluoride (“PVDF”) and polyethersulfone (“PES”), so that they possess both hydrophilic and low protein binding properties in addition to the beneficial characteristics of the material as described above. In summary, the most commonly employed methods for modifying polymeric membranes may be described as follows: (i) polymeric coating with crosslinking, (ii) surface activation followed by grafting and (iii) blending with co-polymers.
An important advantage to modifying a membrane using the method involving polymer coating with crosslinking is its simplicity, both conceptually and in practice. However, a disadvantage to currently used techniques is that the applied coating may not be thermally or mechanically stable. Surface modifications, such as hydrophilicity, which are imparted to hydrophobic membranes by this method are prone to deterioration over time. This is especially true when these membranes are exposed to high temperature conditions.
U.S. Pat. No. 4,618,533 (the '533 patent) describes a typical coating method where a PVDF membrane is post-treated with a solution containing a monomer, crosslinker and initiator prior to ultraviolet radiation (UV) or thermal treatment. The '533 patent teaches using the monomers hydroxyalkyl acrylate or methacrylate, of which, hydroxypropyl acrylate (HPA) is an example. A major problem with PVDF membranes fabricated according to the process disclosed in the '533 patent is that they tend to lose part or all of their hydrophilicity after being heating at more than 130° C.
Methods involving surface activation followed by grafting requires exposure to UV or electron beam (EB) radiation, high energy chemicals or other high energy source, such as ozone or plasma, to generate free radicals or other highly activated species on the substrate surface for grafting. For example, in U.S. Pat. Nos. 5,019,260 and 5,736,051, high energy electron beams or gamma rays are used to activated the surface before the membrane contacts the monomer solution, and without such high energy irradiation, no reaction will occur.
Although the grafting method usually preserves most of the substrate properties and generally provides the best overall results out of the three methods for modifying a membrane listed above, it is not widely used because of the expense and safety issues associated with using high energy sources or high energy chemicals. Because of these problems, this technique is typically reserved for rare instances in which the substrate material can be activated rather easily.
Blending with copolymers is a less rigorous and more economical method for modifying membrane properties. However, this technique is also less effective than the aforementioned methods because the co-polymers added to modify the membrane characteristics are dispersed throughout the resulting membrane rather than being congregated along surface, which would yield the greatest advantages of the modification. Thus, much of the benefits from the blending are lost. Another disadvantage to this method involves the co-polymer itself which must be synthesized, isolated and purified prior to being blended. Another problem with blending is that it may compromise the structure of the original polymer membrane, limiting the useful concentration range, and therefore making it difficult to modify the formulation in response to process changes.
For example, disclosed in WO98/08595, and later published in Macromolecules 32: 1643–1650 (1999) is the blending of comb co-polymers with PVDF to make casting dope. As described above, membranes produced from the such a method normally possess lower degrees of wettability and porosity as compared to membranes produced by the other aforementioned methods. Furthermore, the polymerization reaction and purification must be conducted before the polymer can be blended with PVDF.
Further examples of copolymer blending are found in U.S. Pat. Nos. 5,066,401 and 4,302,334, which generally disclose processes for forming hydrophilic membranes by hydrolyzing a blend of PVDF and a second polymer, such as polyvinyl acetate. Membranes from such processes exhibit better wettability in acid solution, primarily due to further hydrolysis being catalyzed under the acidic conditions. However, in neutral condition, the wettability is inferior. A similar kind of blending process was described in U.S. Pat. Nos. 5,122,273 and 5,503,746, in which polyalkylene glycol or polyvinyltrifluoroacetate were used. Although membranes from such process exhibit reduced protein binding, the reduction is not enough to make them suitable for protein solution filtration, among other things.
In summary, the presently available methods for modifying membranes to impart desirable characteristics thereto have many disadvantages. The problems associated with these methods notwithstanding, there remains a strong demand for improved membranes for a variety of existing applications. Furthermore, there are current and possibly future situations in which membranes could be utilized advantageously but are not because of the problems associated with membranes produced by the aforementioned methods. The purpose of the present invention is to provide a useful process for modifying membranes and a modified membrane, which, among other things, overcomes the shortcoming of the prior art.